In a chain transmission apparatus used in the case where rotation of a crankshaft of the engine is transmitted to a cam shaft or the like, in general, a proper tension is applied to a chain by a tensioner to suppress vibrations generated in the chain during operation.
In a conventional tensioner, there is a ratchet type tensioner with a hydraulic buffer mechanism into which are incorporated a hydraulic check valve mechanism for buffering an excessive tension drastically exerted on the chain when operation starts or the like, and a ratchet mechanism for coping with an elongation which may result after a lapse of time in operation of the chain.
FIGS. 6A and 6B show one example of a ratchet type tensioner with a hydraulic buffer mechanism. A tensioner 21 is arranged, for use, in the vicinity of the upper end of a tensioner lever 26 having a lower end supported by a pivot shaft 25 and extending pivotally along the outer periphery on the slack run of a chain 24 stretched between a driving sprocket 22 and a driven sprocket 23. The tensioner 21 has a plunger 27 for pressing against the upper end of the tensioner lever 26 to apply tension to the chain 24 through the tensioner lever 26.
As shown in FIG. 6B, the plunger 27 comprises an outer plunger 29 held to be moved in and out of a plunger housing 28 and whose backward movement is controlled by a ratchet mechanism L, and an inner plunger 30 slidably inserted therein.
The outer plunger 29 is urged in a projecting direction with respect to the plunger housing 28 by a plunger spring 31, and the inner plunger 30 is urged in a projecting direction by an inner plunger spring 32 having a larger spring constant than the plunger spring 31, the inner plunger 30 being interiorly formed with an oil reservoir chamber 30A in communication with a high pressure chamber H through a check valve 33.
The check valve 33 prevents oil from flowing to the oil reservoir chamber 30A from the high pressure chamber H. When a shock load is exerted on the projecting end of the inner plunger 30 from the tensioner lever 26, oil filled in the high pressure chamber H leaks outside the high pressure chamber H through a restricted clearance provided by the slidable engagement between the inner peripheral surface of an inner plunger receiving bore 29A and the outer peripheral surface of the inner plunger 30 to buffer the shock load.
FIGS. 7A and 7B show a further example of a tensioner having a ratchet mechanism and a hydraulic valve mechanism. In this tensioner 21', a tensioner lever 26' is arranged along the outer periphery of the slack run of a chain 24' stretched between a driving sprocket 22' and a driven sprocket 23', as shown in FIG. 7A. The tensioner lever 26' has the upper end supported by a pivot shaft 25', and the tensioner 21' has an obliquely downwardly-directed plunger 27' for pressing against the lower end of the tensioner lever 26'.
As shown in FIG. 7B, the plunger 27' is composed of an outer plunger 29' and an inner plunger 30', similar to the plunger 27 of the tensioner 21 shown in FIGS. 6A and 6B, and is held to be moved in and out of a plunger housing 28'.
The outer plunger 29' is urged in a projecting direction by a plunger spring 31', and the backward movement thereof is prevented by a ratchet mechanism L'. Further, the inner plunger 30' has a closed-end tubular bore 30'B open at the opposite end to the bottom of an inner plunger receiving bore 29'A of the outer plunger 29'. Into the tubular bore 30'B are slidably inserted an inner plunger spring 32' having a larger spring constant than the plunger spring 31', a check valve plug 33', and a hollow sleeve 34' in sequence from the closed end to the opposite and in contact with one another.
In the tensioner 21', a high pressure chamber H' is formed between the bottom of the tubular bore 30'B and the check valve plug 33', an oil reservoir chamber is formed by a space within the hollow sleeve 34', and a check valve provided in the check valve plug 33' controls a flow of oil from the high pressure chamber H' to the oil reservoir chamber.
When a shock load is exerted on the projecting end of the inner plunger 30' from the tensioner 26', oil filled in the high pressure chamber passes through a clearance between the inner peripheral surface of the tubular bore 30'B and the outer peripheral surface of the check valve plug 33' and leaks toward the oil reservoir chamber so as to buffer the shock load.
In the above-described tensioners shown in FIGS. 6 and 7, when an elongation occurs in the chain after operation for a long period of time, the outer plunger advances along with the inner plunger by the urging force of the plunger spring to prevent slack in the chain due to the elongation.
At that time, since a retractable stroke of the inner plunger is always limited to a given amount, the chain is not excessively loosened due to the retracting displacement of the inner plunger.
In the above-described tensioners shown in FIGS. 6 and 7, the outer plunger is interior of the inner plunger receiving bore, and the inner plunger spring is interior of both the outer plunger and the inner plunger. Therefore, there poses a problem in that the construction become complicated and the manufacturing cost is high. Further, since the high pressure chamber is formed within both the outer plunger and the inner plunger, it has been difficult to provide an oil path for introducing oil into the high pressure chamber to restore the oil passing through the clearance.
Further, when a shock load is exerted on the inner plunger from the chain run, the shock load is transmitted to the outer plunger through oil in the high pressure chamber. Therefore, a large load is applied to the ratchet mechanism which prevents the retraction of the outer plunger to possibly cause damage or early wear in the ratchet pawls and ratchet teeth constituting the ratchet mechanism.